In the coating process of the automotive industry, the yield of the paint sprayed to an automobile body is generally from 60 to 80% and 40 to 20% of the used paint constitutes a surplus paint to be removed in the subsequent process. For the collection of the surplus paint sprayed excessively, treatment is usually applied in a wet spray booth by water washing and the washing water is circulated for re-use.
In the wet paint spray booth constructed as described above, since the surplus paint which is collected in the washing water has high tackiness, it adheres to the water-film panel, the piping system, the spray nozzle, etc. of the spray booth, clogs the piping system and the nozzle, and seriously degrades the efficiency of water washing. Further, when clogging by the deposition of the surplus paint occurs in a gas discharge system of the spray booth, and the flow of air is inhibited, the solvent of the paint fills up the interior of the booth to produce a state dangerous to safety and hygiene and seriously degrade the working environment. Moreover, the greater part of the surplus paint settles to the bottom of the booth pit and the bottom of the circulation pit in the spray booth and the sedimented paint hardens into a rubbery state with elapse of time to take much time and labor for the cleaning and removal.
For the solution of these problems, paint spray booth treating agents to be added to the washing water have hitherto been developed, and methods of adding chemicals such as:
(1) alkalis such as NaOH and KOH, PA1 (2) amphoteric metals such as Zn salts, PA1 (3) cationic polymers, and PA1 (4) surface active agents PA1 (a) the effects are not sufficient, PA1 (b) the applicable pH ranges is narrow, stable treatment is difficult, and it requires a great amount of pH adjusting agents, PA1 (c) the kind of paints applicable is limited and the effect is low for acrylic paints and phthalic acid type paints, PA1 (d) COD in the circulating water is increased due to liberation of treating agents, and PA1 (e) the amount of the paint sludge is increased in the case of using the metal salt, and they can not considered advantageous from an industrial point off view. PA1 (f) In a case where paint load is great, sufficient reduction of tackiness may sometimes be impossible, and PA1 (g) effect for the reduction of tackiness is scarcely obtained for a raw paint.
to the washing water, alone or in a combined use of two or more of them have been known. In addition, there has been proposed (5) a method of using melamine--aldehyde acid colloid (Japanese Patent Publication Sho 63-33799, corresponding to U.S. Pat. No. 4,656,059).
Among the conventional treating agents mentioned above, (1) to (4) entail the disadvantages, for example,
Further, any of (1)-(5) involves the following drawbacks when used alone.
The paint load means the charging rate or flow velocity of a surplus paint to be treated (over sprayed paint) and it is usually about 100 ppm based on the amount of circulating water (amount of paint/amount of circulating water) and about 200 ppm at the greatest, which is different depending on the types of the paint used. Due to the introduction of manipulators and increase in the number of products in recent years, the amount of the surplus paint tends to be far increased relative to the capacity of the facility, which increases the paint load. Further, the raw paint requires treatment, for example, upon color-change (change from a certain color to another color). That is, upon color-change, since the remained paint in the piping system from the paint reservoir to the spray gun is often discarded without spraying in to the washing water, its treatment becomes necessary. While the raw paint is recovered depending on the case but the complete recovery is difficult and, even in the case of recovery, at least a portion thereof has to be treated.
Further, Japanese Patent Laid-Open Sho 63-260970 discloses an invention of adding, for example, glutamic acid to a booth circulating water. Further, Japanese Patent Laid-Open Sho 62-298478 describes an invention of adding, for example, a maleic acid series polymer but satisfactory effects can be obtained by none one of such methods.